Trade Space Analysis of Receiver Architectures with Non-Linearity Performance Evaluation for Digital Microwave Radiometers in Radio-Frequency Interference Environment

Abstract

Microwave radiometers are highly sensitive receivers that remotely measure the average power of natural thermal emissions from the Earth’s surface and its atmosphere. This measured power is converted to brightness temperature to estimate different geophysical parameters. The man-made signals like radars, communication systems, air traffic control, etc. cause interference for radiometric measurements and are known as radio-frequency interference (RFI). The RFI threatens the authenticity of the measured radiometric data since it corrupts the measurements. Different types of RFI detection and mitigation algorithms have been developed to date. Linearity is a critical performance parameter for RF communication systems. The linearity performance of microwave radiometers is more significant since radiometers are designed to be sensitive receivers to measure low levels of thermal emissions. This feature makes the radiometer susceptible to RFI signals that can further impact the linearity of the radiometer by generating additional spurious signals. Therefore, the first contribution of this research work offers a quantitative analysis of non- linearity performance of a microwave radiometer for a CW-type of RFI signal. The radiometer’s performance is evaluated for both second and third-order non-linearity. The results of the spur analysis with the Soil Moisture Active Passive (SMAP) radiometer system specifications confirm that spur power level is not significant to contribute to in-band fundamental RFI signal. This suggests that the non-linearity specifications can be relaxed for a radiometer. The second outcome of this research is the derivation of equations to determine the output second-order intercept point (OSOI) and output third-order intercept point (OTOI) specifications for a microwave radiometer based on the science requirements and RFI environment. These developed equations can assist system designers to determine the non-linearity specifications based on the science requirements and system specifications for a given RFI environment. Since superheterodyne is the conventional receiver architecture implemented in microwave radiometers, therefore, it is important to explore the benefits and limitation of other receiver architectures over the heterodyne receiver architecture. Therefore, trade-off study between superheterodyne and direct conversion receiver architecture has been performed for a microwave radiometer. The heterodyne architecture was determined to be more favorable receiver architecture than the direct conversion for a radiometer based on discussed figures of merits.